Allow implicit self for weak self captures, after self is unwrapped

Paul Hudson      @twostraws

SE-0365 takes another step towards letting us remove self from closures by allowing an implicit self in places where a weak self capture has been unwrapped.

For example, in the code below we have a closure that captures self weakly, but then unwraps self immediately:

class TimerController {
    var timer: Timer?
    var fireCount = 0

    init() {
        timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] timer in
            guard let self else { return }
            print("Timer has fired \(fireCount) times")
            fireCount += 1
        }
    }
}

That code would not have compiled before Swift 5.8, because both instances of fireCount in the closure would need to be written self.fireCount.

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Other changes in Swift 5.8…

• Lift all limitations on variables in result builders
• Function back deployment
• Concise magic file names
• Opening existential arguments to optional parameters
• Collection downcasts in cast patterns are now supported

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